Fire detecting apparatus sensitive to refraction

ABSTRACT

Fire detecting apparatus comprises an emitter for generating a beam of electromagnetic radiation and a detector for producing an output signal dependent upon the amount of radiation incident thereon. The apparatus is arranged to be responsive to a fluctuating component in the output of a detector to operate an alarm or fire control means.

United States Patent 91 Lawson et al.

11 3,723,746 1 Mar. 27, 1973 FIRE DETECTING APPARATUS SENSITIVE TOREFRACTION Inventors: Dennis Illingworth Lawson; Eugene FrancisOSullivan, both of Elstree, England National Research DevelopmentCorporation, London, England Filed: Jan. 7, 1971 Appl. No.: 104,675

Assignee:

Foreign Application Priority Data Great Britain ..857/70 Great Britain..34,2 1 8/70 Jan. 7, 1970 July 14, 1970 US. Cl ..250/2l8 R, 250/2l7 F,250/220, 250/237, 340/228.2, 356/207 Int. Cl. ..G0ln 21/26 Field ofSearch ..250/2l7 F, 204, 220, 237, 218, 250/205; 340/228.2, 237;356/124, 207

[56] References Cited UNITED STATES PATENTS 2,404,147 7/1946 Strickland..250/205 3,566,385 2/l97l Lawson ..356/207 Primary Examiner.lames W.Lawrence Assistant Examiner-D. C. Nelms Attorney-Cushman, Darby &Cushman [57] ABSTRACT Fire detecting apparatus comprises an emitter forgenerating a beam of electromagnetic radiation and a detector forproducing an output signal dependent upon the amount of radiationincident thereon. The apparatus is arranged to be responsive to afluctuating component in the output of a detector to operate an alarm orfire control means.

12 Claims, 6 Drawing Figures PATENIEU N [975 3 723 746 SHEET 2 OF 2 FIREDETECTING APPARATUS SENSITIVE T REFRACTION The invention herein relatesto improvements in fire detection apparatus as disclosed in our U.S.Pat. No. 3,566,385 for Fire Detecting Apparatus Sensitive to Refraction,and the applications of OSullivan et al. Ser. No. 162,126, and Burry,Ser. No. 162,167, both filed July 13, 1971.

According to the present invention there is provided fire detectingapparatus comprising emitter means for generating a beam ofelectromagnetic radiation, photodetector means, sensitive to the saidradiation, and providing an output signal dependent upon the radiationfalling thereon, and control means responsive to a fluctuating componentin the output signal of the detector means for producing a controlsignal. The apparatus may be adapted to operate alarm and/or firecontrol means in response to detection of the said fluctuating componentresulting from disturbances of the beam of radiation by a fire in thespace through which the beam passes.

The photo-detector means may comprise a photosensitive surface inregister with a mask bearing a checker board pattern with alternatesquares of the checker board opaque and radiation transmitting, the beamof radiation being arranged to fall on the mask so that movement of thebeam on the mask produces a fluctuating output signal at the photosensitive surface.

The term checker board pattern as used herein, means a two-dimensionalarray comprising rows of alternate active and inactive areas. Adjacentrows are displaced with respect to one another so that an active elementin one isle is aligned with inactive elements in the two immediatelyadjacent rows. In the case of a mask, the active elements aretransparent to radiation and the inactive elements are opaque.

As an alternative, the mask and photocell arrangement may be replaced byan array of discrete photocells mounted in a checker board pattern on asupport, with the space between the cells of the same order as the areaof the cells. The outputs of the cells are summed, so that the movementof the beam spot over the array of cells in the presence of a fireproduces a fluctuating output from the sum of the cell outputs.

Alternatively the photo-detector means may comprise a single photocelland the beam of radiation may be directed to fall on or adjacent thephotocell in such a manner that occurrence of a fire in the spacethrough which the beam passes between the emitter means and the detectormeans causes the beam to oscillate over a boundary of the photocell sothat the output of the cell fluctuates.

It is preferred that the electromagnetic radiation is in the visible ornear visible range of wavelength, and preferably the emitter is a laser.Alternatively the emitter means may include a semiconductorlightemitting diode in the infra-red or visible ranges, and theradiation used may be modulated with the various detecting circuitsresponsive only to the frequency of modulation.

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic plan view of fire detecting apparatus accordingto the present invention;

FIG. 2 is a perspective view of part of the fire detecting apparatus ofFIG. 1;

FIG. 3 is a perspective view of a modification of the part of theapparatus shown in FIG. 2;

FIG. 4 is a diagrammatic perspective view of an alternative form of firedetecting apparatus according to the invention;

FIG. 5 is a perspective view of a corner-cube reflector for use with theapparatus of FIG. 4; and

FIG. 6 illustrates a modification of the apparatus shown in FIG. 4 toprovide an intruder alarm;

Referring to FIGS. 1 and 2, a fire detecting apparatus comprises anemitter 10 which directs a beam 11 of electromagnetic radiation across aspace 12. There is provided a beam detector 13 which, if the beam is inthe visible range of wavelengths, may be a photocell, the detectorproducing an output dependent upon the amount of illumination thereof bythe beam.

It is preferred that the electromagnetic radiation be in the visible ornear visible range of wavelength since such radiation can be morereadily collimated and thereby employed over large areas by using, forexample, reflection from reflectors. In this connection, it is to benoted that a laser is particularly suitable for providing a highlycollimated beam.

Disposed between the emitter 10 and the detector 13 is a mask 14 whichis shown in greater detail in FIG. 2 This mask 14 is of checker boardconfiguration and has, in the particular example shown in FIG. 2, equalnumbers of transmitting and absorbing portions which respectively allowor prevent the beam 11 falling on the detector 13.

The output from the detector 13 is amplified by an amplifier 14 which,is connected to and is adapted to actuate alarm and/or fire controlmeans 15. The alarm and/or fire control means may provide an audibleand/or visible warning of fire and may be operated locally or remotefrom the remainder of the fire detecting apparatus.

In the arrangement of FIG. 1 it will be appreciated that the beam 11 ispermitted by the mask 14 to illuminate the detector 13. Fluctuations inthe position of the beam 11 caused by, for example, changes in theambient conditions in the space 12 may cause the beam to fall uponabsorbing portions of the mask which prevent it from illuminating thedetector 13. This will produce a slow change in the output from thedetector 13 but the amplifier 114' is so arranged that these slowfluctuations in the output in the detector 13 'do not cause actuationsof the alarm and/or fire control means 15.

However, if there is a fire present in the space 12, more rapidfluctuations in the position of the beam 11 will cause the same to fallalternately on the transmitting and absorbing portions of the mask 14,and thus the beam 11 will alternately illuminate and be prevented fromilluminating the detector 13. The amplifier I4 is so arranged that theresultant alternating output from the detector 13 causes actuation ofthe alarm and/or fire control means.

This arrangement has the advantage that the fire de tecting apparatus isnot affected by changes in ambient conditions within the space 12 andthe alarm and/or fire control means are not actuated. However, thelatter are actuated by the presence of fire in the space 12.

Referring now to FIG. 3 there is shown a modified form of mask 17 tothat shown in FIG. 2. The mask 17 comprises a plurality (25 shown) ofcollimating elements 18 through which the beam 11 may pass andilluminate the detector 13. There are also provided a plurality (24shown) of portions 19 which prevent the beam 11 from illuminating thedetector 13. The purpose of the collimating portions 18 is substantiallyto prevent the detector 13 from receiving radiation other than the beam11.

By way of example, the variations in refractive index of air in a fireare of the order of one per million per degree Celsius. A temperaturegradient of 100 C per meter, for example, would cause the beam to followa circular path of km in radius and after transversing about 100 m, thedeflections from fires are likely to be of the order of 10 mm. If adeflection of this magnitude is to be received and recorded by aphotocell, it is necessary for the beam to be highly collimated. Thespot of light after focusing the beam, should preferably not be greaterin linear dimensions than the deflection of the beam caused by the fireand a laser beam is preferable for this degree of collimation.

Under no-fire conditions a laser beam may be focused to form a spot at aremote distance by means of a telescope. The spot has a diameter (1.2/a)1, where A is the wavelength of the laser (0.6328 microns for thehelium-neon laser) a is the diameter of the objective used to form theimage, and l is the distance of the laser from the spot. Over a distanceof 100 m and with an objective of diameter 50 mm, the spot would have adiameter of 1.2 X 0.6328 X 10"")l50 X 10 mm, i.e., of the order of 1 mm.The theoretical spot size is difficult to attain because of lensimperfections and 5 mm is a practical figure at this distance.

Even under quiet conditions the spot exhibits a slight movement of theorder of a millimeter over a path of about 100 m.

Once a fire is lit under or to one side of the beam, the spot becomesviolently agitated after a few seconds, the time lag depending on theheight of the ceiling above the fire. This violent movement is aconsequence of the turbulent plume of gas from the fire impinging on theceiling and spreading out in much the same way as a torrent of watermight strike and spread out over a floor. The turbulence causes the beamto deflect in a random manner about its rest position. If the fireprogresses, the space immediately under the ceiling fills with hot gas,rather as a bath fills with water, and the laser beam traverses a layerof hot gas which is now less turbulent, the turbulent region at thistime having moved below the beam. The agitation of the spot is lessviolent and it moves as a whole from its original undisturbed condition.As this may take considerably longer than the first indication ofturbulence in the spot, it is important to develop a detection systemusing the turbulent movement of the spot. The deflection of the beam d,would be expected to fall off with height of compartment z, according tothe law dar The size of fire necessary to operate the detector increasesproportionally with z.

If a simple detection system is constructed in which the beam from thelaser traverses the space to be protected and is then received by aphotocell, it suffers from the disadvantage of the beam being moresensitive to fires near to the laser than near to the photocell becauseof the optical lever effect.

The efiect can largely be overcome by locating a mirror at the end ofthe protected space to return the beam on to a photocell placed near tothe laser, so that the sensitivity due to the optical lever should nevervary by more than a factor of two.

Referring to FIG. 4, an emitter means 21 directs a beam ofelectromagnetic radiation 22 along the ceiling of a room 23 to a mirror24 which reflects the beam back to photo-detector means 25. Thephoto-detector 25 can take any of the forms previously described. Thisarrangement reduces the variation in sensitivity of the apparatus withdistance of the fire from the laser but raises the problem of thestability of the mirror since any slight angular change of the reflectedbeam due to movement of the mirror mounting is magnified and recorded bythe photocell as an alarm. This can be overcome by using a so-calledcomer-cube mirror system. As shown in FIG. 5, this consists of threemirrors 26, 27 and 28 disposed mutually at right angles with theirreflecting surfaces directed inwards. Any ray of light incident on thereflecting surface of one of the mirrors is returned on a path parallelto the incident path irrespective of the orientation of the mirrorsystem, subject only to the limitation that the angle of incidence doesnot differ from that indicated by the arrow A to such a large extentthat the path of the reflected light would intercept one of the mirrorplanes beyond the boundary of that mirror.

There is always a small amount of fluctuation of the beam caused byambient changes arising from convection in the area to be protected, butthese occur at a lower frequency than those associated with fires. Thus,if the amplifier is tuned to receive a frequency of 40 Hz, it ispossible to discriminate against ambient fluctuations. A delay is alsoincorporated into the system to prevent false alarms, should the beam bemomentarily interrupted.

Referring to FIG. 6, part of the beam from laser 21 is diverted, bymeans of a half-silvered mirror 29, on to a reflector 30 just abovefloor level. The reflector 30 diverts the diverted part of the beam 31across the room 23 and on to a further reflector 32 and thence to aphotocell 33 which is used to control operation of an intruder alarm.The laser beam is invisible until viewed almost along the line of thebeam and consequently it is unobtrusive.

By way of example, a system using a helium-neon laser installed in abuilding 41m X15 m X12 m high ftX SOftX 40fthigh) canbe arranged todetect a fire (711 mm (28 in) in diameter) in liquid fuel at any pointin the building within half-a-minute.

We claim: 1. Fire detecting apparatus comprising: emitter means forgenerating a constant and stationary beam of electro-magnetic radiation,

photodetector means sensitive to said radiation and producing an outputsignal dependent upon the position of the beam of radiation incidentthereon from said emitter means, and

control means responsive to an alternating component having a firstpredetermined frequency range and non-responsive to an alternatingcomponent having a second predetermined frequency range in the output ofthe photodetector means for producing a control signal.

2. Fire detecting apparatus as claimed in claim 1, in which thepredetermined range is 40 to 70 Hz.

3. Fire detecting apparatus as claimed in claim 1, including an alarmarranged to operate in response to said control signal.

4. Fire detecting apparatus as claimed in claim 1, including firecontrol means arranged to operate in response to said control signal.

5. Fire detecting apparatus as claimed in claim 1, in which thephotodetector means comprises: a photocell and a mask disposed betweenthe emitter means and said photocell, said mask having transmitting andabsorbing portions which respectively allow and prevent said beamilluminating said detector means and which are arranged in a checkerboard configuration, whereby fluctuations in the position of said beamcause the beam to alternately illuminate the respective portions of themask thereby causing the output from the photocell to fluctuate.

6. Fire detecting apparatus as claimed in claim 5, in which eachtransmitting portion of the mask comprises collimator means.

7. Fire detecting apparatus as claimed in claim 1, in which thephotodetector means comprises a plurality of discrete detectors arrangedin an array with spaces between adjacent detectors of substantially thesame area as the area of each detector and means for adding the outputsof the detectors to form a single output for the detector means.

8. Fire detecting apparatus as claimed in claim 1, in which thephotoconductor means comprises a single detector of substantially thesame area as the area of cross-section of the beam of radiation at thedetector, the emitter means being arranged to detect the beam on to theedge of the detector in such a manner that occurrence of a tire in thespace through which a beam passes between the emitter means and thephotodetector means causes the beam to oscillate over a boundary of thedetector.

9. Fire detecting apparatus as claimed in claim 1, in which thephotodetector means is disposed adjacent to the emitter means, reflectormeans comprising three adjacent mirrors disposed at right angles to oneanother, being arranged to reflect the beam of radiation on to thephotodetector means.

10. Fire detecting apparatus as claimed in claim 1, in which ahalf-silvered mirror is disposed in front of the emitting means todivert a portion of the beam, said portion being directed on tosubsidiary detector means which is arranged to operate an intruderalarm.

11. Fire detecting apparatus as claimed in claim 1, in which the emittermeans is arranged to generate radiation in the visible range.

12. Fire detecting apparatus as claimed in claim 1 l in which theemitter means is a laser.

1. Fire detecting apparatus comprising: emitter means for generating aconstant and stationary beam of electro-magnetic radiation,photodetector means sensitive to said radiation and producing an outputsignal dependent upon the position of the beam of radiation incidentthereon from said emitter means, and control means responsive to analternating component having a first predetermined frequency range andnon-responsive to an alternating component having a second predeterminedfrequency range in the output of the photodetector means for producing acontrol signal.
 2. Fire detecting apparatus as claimed in claim 1, inwhich the predetermined range is 40 to 70 Hz.
 3. Fire detectingapparatus as claimed in claim 1, including an alarm arranged to operatein response to said control signal.
 4. Fire detecting apparatus asclaimed in claim 1, including fire control means arranged to operate inresponse to said control signal.
 5. Fire detecting apparatus as claimedin claim 1, in which the photodetector means comprises: a photocell anda mask disposed between the emitter means and said photocell, said maskhaving transmitting and absorbing portions which respectively allow andprevent said beam illuminating said detector means and which arearranged in a checker board configuration, whereby fluctuations in theposition of said beam cause the beam to alternately illuminate therespective portions of the mask thereby causing the output from thephotocell to fluctuate.
 6. Fire detecting apparatus as claimed in claim5, in which each transmitting portion of the mask comprises collimatormeans.
 7. Fire detecting apparatus as claimed in claim 1, in which thephotodetector means comprises a plurality of discrete detectors arrangedin an array with spaces between adjacent detectors of substantially thesame area as the area of each detector and means for adding the outputsof the detectors to form a single output for the detector means.
 8. Firedetecting apparatus as claimed in claim 1, in which the phOtoconductormeans comprises a single detector of substantially the same area as thearea of cross-section of the beam of radiation at the detector, theemitter means being arranged to detect the beam on to the edge of thedetector in such a manner that occurrence of a fire in the space throughwhich a beam passes between the emitter means and the photodetectormeans causes the beam to oscillate over a boundary of the detector. 9.Fire detecting apparatus as claimed in claim 1, in which thephotodetector means is disposed adjacent to the emitter means, reflectormeans comprising three adjacent mirrors disposed at right angles to oneanother, being arranged to reflect the beam of radiation on to thephotodetector means.
 10. Fire detecting apparatus as claimed in claim 1,in which a half-silvered mirror is disposed in front of the emittingmeans to divert a portion of the beam, said portion being directed on tosubsidiary detector means which is arranged to operate an intruderalarm.
 11. Fire detecting apparatus as claimed in claim 1, in which theemitter means is arranged to generate radiation in the visible range.12. Fire detecting apparatus as claimed in claim 11 in which the emittermeans is a laser.